In vapor compression, air-conditioning or heat pump systems, a heat exchanger is disposed in some sort of an air handling device as a plenum associated with a blower. In the case of air-conditioning systems, most usually, but not always, the plenum is the hot air plenum of a furnace.
In any event, in both air-conditioning systems and in heat pump systems, for cooling purposes this heat exchanger is employed as an evaporator to evaporate a working fluid for the purpose of absorbing heat from ambient air driven through the heat exchanger by the blower associated with the plenum. In the case of a heat pump, this heat exchanger, for heating purposes, will also serve as a condenser for the working fluid which gives up heat to the ambient air upon condensing within the heat exchanger.
Most frequently, these heat exchangers are in the form of so-called A-coil evaporators. In the usual case, an A-coil evaporator is in actuality formed of two essentially separate heat exchangers that are inclined upwardly toward each other. Each heat exchanger is made up of a plurality of plate fins disposed in parallel and in vertical planes through which horizontally oriented, round tubes pass. The ends of the round tubes emerging at each end of the bundle of plate fins are connected by U-tubes. A double trough which is in effect a trough with an open center mounts the two heat exchangers with the trough on each side of the central opening receiving condensate from an associated one of the two heat exchangers. The air to be cooled or heated passes up the open center of the trough and through the plate fins in an upward and diagonally outward direction within the plenum.
While A-coil evaporators have worked well for their intended purpose, they are not without a number of drawbacks. For one, in many instances when operating as evaporators, condensate condensing on the plate fins does not drain well and may bridge the gap between adjacent fins.
The resulting water bridge impedes air flow through the heat exchanger which in turn reduces heat transfer from the air. Fin and tube temperature may drop such that the water bridging the fins begins to freeze. As a consequence, drainage is increasingly impeded and the entire heat exchanger may ultimately freeze up.
While this difficulty may be solved by employing a greater air flow, that results in increased capital expense in terms of a larger motor and/or blower as well as increased operating costs.
Furthermore, manifolding the two heat exchangers together into an A-coil heat exchanger requires the performance of a significant number of purely manual operations, thus increasing manufacturing costs. In addition, because each tube has two soldered or brazed joints, one at each end, there is a relatively large potential for refrigerant leakage where a large number of tubes are employed. Conventional A-coils are also heavy and frequently difficult to handle during installation as a result. They are also easily damaged.
In many instances, proper distribution of the refrigerant through the heat exchanger when used as an evaporator may require fairly complex plumbing and/or a complex distribution system to achieve a desired degree of temperature uniformity of exiting air across the heat exchanger.
Moreover, when a heat exchanger is employed in a heat pump system, and thus must function as both an evaporator and a condenser, efficiencies are of substantial concern. Two phase heat exchange operations as when one heat exchange fluid is transitioning from the liquid phase to the vapor phase or vice versa are nowhere near as well understood as single phase heat exchange operations. Furthermore, in differing two phase heat exchange operations, what may be of concern in one is not of concern in the other. For example, in the two phase heat exchange operation of evaporation within a refrigeration system, provision must be made to dispose of condensate resulting from moisture in ambient air being passed through the heat exchanger condensing on the cool surfaces thereof. There is no corresponding air side problem in the two phase operation of condensation. However, in the two phase operation of condensation on the refrigerant side, gravitational effects may dictate the orientation of various passages because it is difficult to cause condensate to flow uphill when mixed with substantially less dense vapor. In heat exchangers operating as evaporators, orientation for gravitational purposes may not be as critical.
On the other hand, if incoming refrigerant in a heat exchanger operating as an evaporator is not uniformly distributed, substantial temperature differentials in air exiting different parts of the evaporator may occur and heat exchange efficiency suffers. Distribution of incoming vapor is not, however, a great concern in a heat exchanger operating as a condenser.
Consequently, there is not only a real need for a new and improved evaporator, there is also a need for a heat exchanger that can alternatively function with great efficiency as both an evaporator and as a condenser so as to be especially adapted for use in a heat pump system.